In radio access networks, it is commonplace to have procedures for a User Equipment device (UE) to reestablish connection to a server (e.g., a radio access node) if a connection between the UE and the radio access network is lost. The loss of the connection is typically due to poor radio conditions, e.g., low signal strength, high interference, or both. In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, one such procedure is the so-called Radio Resource Control (RRC) Connection Reestablishment procedure as defined in Section 5.3.7 of 3GPP Technical Specification (TS) 36.331 version 11.9.0 (Release 11). One trigger for initiating an RRC Connection Reestablishment is for the UE to declare a Radio Link Failure (RLF). An RLF is declared if the UE deems that the radio conditions are poor enough that reliable reception of the downlink control channel (i.e., the Physical Downlink Control Channel (PDCCH)) is not possible.
FIG. 1 illustrates one example of an RRC Connection Reestablishment for a UE that declares an RLF in a single-band scenario (i.e., there is only one frequency band used by all cells). As illustrated, in this example, the UE is on the edge of Cell A moving toward Cell B. Before a handover is initiated to handover the UE from Cell A to Cell B, or before handover is completed successfully, the UE declares an RLF while still being served by Cell A. Upon RLF declaration, the UE initiates a cell search and, in this example, the UE reselects Cell B since, in this example, the UE measures better signal strength/quality for Cell B as compared to Cell A. The UE then attempts an RRC Connection Reestablishment to Cell B. In this case, Cell B is referred to herein as a target cell for the RRC Connection Reestablishment and Cell A is referred to herein as a source cell. After executing a random access procedure to Cell B (the newly reselected cell), the UE sends an RRC Connection Reestablishment Request message containing a Physical Cell Identifier (PCI) to Cell A (i.e., the previous serving cell of the UE, which is also referred to as the source cell) and a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the UE when the UE was being served by Cell A. In this example, Cell A has a PCI value of X.
The C-RNTI uniquely identifies the UE while the UE is being served by a given cell, and is used for several purposes including addressing the user when downlink data is available for transmission. According to 3GPP standards, the C-RNTI is a 16-bit value, meaning 32,768 values are available. Every cell uses the same set of C-RNTIs, and it is up to the implementation on how the C-RNTIs are allocated and reused as UEs go in and out of RRC_CONNECTED state. With this scheme, the C-RNTI used for a UE in a given cell has meaning within that cell. The same C-RNTI could be used in another cell to address a different UE.
Assuming that Cell B has not already obtained the context of the UE through handover preparation or other methods, Cell B (or more specifically the radio access node (e.g., the enhanced or evolved Node B (eNB)) controlling Cell B) must obtain (fetch) the context of the UE from the previous serving cell (i.e., Cell A) upon receipt of the RRC Connection Reestablishment Request message. The context of the UE is information required in order to continue the data session with Cell B as the serving cell. The UE context contains such information as UE capability, security context, information related to established bearers, etc.
Using current technology, Cell B determines the cell from which to fetch the UE context based on at least two pieces of information, namely: (1) a PCI of Cell A sent by the UE in the RRC Connection Reestablishment Request message and (2) a neighbor list stored at Cell B. The neighbor list contains a list of PCIs mapped to globally unique cell identifiers (i.e., Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Global Cell Identifiers (ECGIs)) of the neighboring cells of Cell B. In the simple example of FIG. 1, the neighbor list of Cell B would contain PCI X and PCI Z mapped to the ECGIs of Cells A and C, respectively. Hence, when Cell B receives PCI X in the RRC Connection Reestablishment Request message, Cell B knows that the UE was previously served by Cell A. In this case, if Cell B was not already prepared for handover, Cell B sends a context fetch request message back to Cell A. At a minimum, the context fetch request message contains the C-RNTI of the UE making the RRC Connection Reestablishment Request so that Cell A can identify the UE for which to send the context back to Cell B. Provided that Cell A has not released the context of the UE (context release occurs after a configurable amount of time), Cell A sends the context of the UE back to Cell B, thereby allowing the RRC Connection Reestablishment procedure to complete.
In some scenarios, the RRC Connection Reestablishment procedure can fail. For example, the RRC Connection Reestablishment procedure may fail if the UE takes too long to access Cell B such that the context of the UE at Cell A is dropped before Cell B has had a chance to fetch the context of the UE from Cell A. Typically, the context of the UE is held for a number of seconds (i.e., long enough for a sizeable fraction of reestablishments to be successful) but not so long as to tie up resources for serving other users.
RRC Connection Reestablishment failure is not catastrophic. The RRC protocol is able to handle such failures. In particular, if failure occurs, the UE initiates a new service request. However, this carries a cost in terms of signaling (e.g., S1 Application Protocol (S1AP) Context Release, Evolved Packet System (EPS) Bearer Update, RRC Connection Setup, etc.), and a longer service interruption time. In the case of Voice over Long Term Evolution (VoLTE) calls, it can lead to call drops. Thus, while the system (i.e., the EPS for LTE) is able to handle the failure of reestablishments, clearly, it is desirable to minimize the number of such failures in order to maintain good radio network performance. As such, there is a need for systems and methods that provide improved success rates for RRC Connection Reestablishments.